Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy.
In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. PA1 2. Image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element. PA1 3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element onto a receptor element.
Thermographic materials of type 1 become photothermographic upon incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
JN 02/00864 discloses a heat-developing photosensitive material comprising a heat-developing photosensitive component which contains at least a photosensitive silver halide, a dye donative material, a reducer and binder on a support and an image-receiving component which is piled with the photosensitive component at least on transferring of image. The developing photosensitive component and/or the image-receiving component contains (a) compound of formula (I): ##STR2## where Y=non-metallic atom group necessary to form a nitrogen-containing heterocyclic; X=S, sulphonyl, OR or quaternary N atom; R=substituent Among the embodiments (A), (B) and (C) of formula (I) given in the description are 1,3,5-triazine compounds and a specific compound according to embodiment (B), compound 9, is 2,4-bis(trimethylammonium)-6-decylamino-1,3,5-triazine-dichloride.
EP-A 831 365 discloses an imaging element for use in an image-forming process; the imaging element comprising a support, an image-forming layer; and a transparent electrically conductive layer comprising polypyrrole/poly(styrene sulfonic acid).
The standard teaching over thermographic materials based on a substantially light-insensitive organic silver salt and a reducing agent for the organic silver salt is that the organic silver salt is formed in an aqueous medium and is precipitated and dried before dispersion in an organic solvent medium from which the dispersion is coated. This production method is very inefficient as the organic silver salt after formation in water has to be separated and dried before dispersion in a solvent medium, is environmentally unsound as evaporation of solvent takes place during the coating process and it involves lengthy utilization of plant during the preparation of the organic silver salt dispersion and coating requires costly plant due to the need for solvent explosion prevention measures and solvent recovery to prevent solvent emission to the environment.
WO 94/16361 addressees this problem and discloses a multilayer heat-sensitive material which comprises: a colour-forming layer comprising: a colour-forming amount of finely divided, solid colourless noble metal or iron salt of an organic acid distributed in a carrier composition; a colour developing amount of a cyclic or aromatic organic reducing agent, which at thermal copy and printing temperatures is capable of a colour-forming reaction with the noble metal or iron salt; and an image-toning agent; characterized in that (a) the carrier composition comprises a substantially water-soluble polymeric carrier and a dispersing agent for the noble metal or iron salt and (b) the material comprises a protective overcoating layer for the colour-forming layer.
WO 95/12495 discloses a method of recording an image by image-wise heating a recording layer, the recording material comprising on the same side of a support, called the heat-sensitive side, (1) one or more layers comprising an imaging composition essentially consisting of (i) a substantially light-insensitive organic silver salt being in thermal working relationship with (ii) a reducing agent, and (2) at same side covering the imaging composition a protective layer, characterized in that the image-wise heating proceeds with a thermal head contacting the heat-sensitive side and through the protective layer mainly comprising a cured polymer or cured polymer composition e.g. hydrophilic polymers having active hydrogen atoms selected from the group of polyvinyl alcohol, partially hydrolyzed polyvinyl acetate and gelatin at least part of which has reacted with hardening agents selected from the group consisting of polyisocyanates, polyepoxides, aldehydes and hydrolysed tetraalkyl orthosilicates.
U.S. Pat. No. 5,661,101 discloses a recording material with, on a support, at least a coloring layer containing a first coloring component which is substantially colorless and a second coloring component which is substantially colorless and is colored by reacting with the first coloring component, and a protective layer provided on the coloring layer and having a pigment and a binder as main components, wherein at least the protective layer contains a polyvinyl alcohol resin having a syndiotacticity of greater than or equal to 55 molar % as diad indication and a saponification degree of greater than or equal to 85 molar %. Furthermore, the protective layer may contain, in addition to the polyvinyl alcohol resin, a cross-linking agent for cross-linking the polyvinyl alcohol resin e.g. epoxy compounds, blocked isocyanates, vinyl sulfone compounds, aldehyde compounds, methylol compounds, boric acid, carboxylic acid anhydrides, silane compounds, chelating compounds and halogenated compounds.
The inventors of the present invention found that prints made with thermographic materials produced from aqueous media, according to the teaching of WO 94/16361, exhibited poor archivability and poor light stability. Furthermore, thermographic materials with crosslinked protective layers coated from aqueous media according to the teaching of WO 95/12495, require the use of substantial quantities of water-miscible solvents, e.g. the use of hydrolyzed tetaalkyl orthosilicates, or involved products such as formaldehyde for which emission norms are extremely low. Furthermore, such thermographic recording materials exhibited poor archivability.